One of the most significant freshwater pearl farms in the world lies on the Tennessee River in the United States.
Because of the large size of these freshwater mussels, many shapes and sizes can be cultured.
Most of the pearls for sale at the gift shop when I visited were coin, pear and tablet shaped. After nucleation these mussels are left do their thing for three to five years before harvesting, yielding a high quality and lustrous cultured pearl because of the extremely thick nacre.
You are probably still wondering why this little farm is so important. The pearl output, in global terms, is minuscule. The significance lies in the shells of these mussels. 95% or more of the world's cultured pearls are nucleated with beads carved from these shells. Thus far, no better nucleus material has been found.